The present invention relates to a steering apparatus for a four-wheel vehicle such as an automobile, which steers front and rear wheels.
In general, a vehicle can be turned upon steering of the front wheels. If rear wheels are also steered in accordance with a steering angle of the front wheels, stability at high and middle vehicle speeds and response at low vehicle speeds can be improved, as is known to those skilled in the art. Conventional mechanical and hydraulic wheel steering apparatuses for steering both front and rear wheels are respectively exemplified in Japanese Pat. Prepublication Nos. 59-26364 and 59-81261. In the former apparatus, a steering force of the front wheels is transmitted to the rear wheels through an actuating shaft coupling the front wheels to the rear wheels. The latter apparatus has a rear wheel power steering unit using hydraulic pressure as auxiliary power and controls an oil flow in accordance with the steering angle of the front wheels.
The conventional mechanical steering apparatus is bulky, however, so that it is difficult to provide adequate mounting space. In the conventional hydraulic steering apparatus, an oil reservoir is located between a rear wheel drive unit and the apparatus drive unit, and the rear wheels are driven by the hydraulic power. This steering apparatus is also bulky and complicated. In addition, a change in oil temperature adversely affects performance, and a countermeasure against oil leakage is required.